Nischit V. Shetty

Evidence for downy mildew races in cucumber tested in Asia, Europe, and North America.

Abstract Downy mildew ( Pseudoperonospora cubensis (Berk. and Curt.) Rostov.) is an important disease in most cucumber ( Cucumis sativus L.) production areas worldwide. A set of cucumber cultivars and breeding lines (hereafter referred to collectively as cultigens) resistant to downy mildew in particular regions of the world were tested for resistance in geographical regions thought to differ in pathogen virulence or race (US, Poland, China, and India). Cucumber cultigens used in the study were developed in the US, Poland or China, and differed in their resistance to downy mildew. These cultigens were evaluated against local isolates of P. cubensis under field conditions (North Carolina and India) or greenhouse conditions (South Carolina and Poland). Significant differences were observed among cultigens for resistance to P. cubensis at all locations. Individual cultigens differed in their resistance to the pathogen at different geographic locations, providing evidence that the different local isolates represented different races of the pathogen. Cultigens from PR China that were resistant to downy mildew in that country were also resistant in India. However, those same cultigens were intermediate in resistance in the US and Poland. Cultigens from the US and Poland that were resistant in those countries were intermediate in resistance in India. The most resistant cultigens over all locations were ‘Nongchen #4’ (PR China) and M 21 (NC State University).